1. Field of the Invention
The present invention relates to a reflecting telescope optical system, an optical unit, and an image pickup apparatus using the same.
2. Description of the Related Art
In the past, a small-size reflecting telescope optical system in which, an overall length of the optical system is shortened by using a main reflecting mirror and a sub reflecting mirror has been known. As a method of focusing in the reflecting microscope optical system, (i) a method of drawing the overall optical system out, (ii) a method of changing a distance of the main reflecting mirror and the sub reflecting mirror, and (iii) a method of moving a rear-side lens unit in an optical axial direction while maintaining a distance of the main reflecting mirror and the sub reflecting mirror to be constant have been known. In a case of carrying out video shooting by a reflecting telescope optical system, a method mentioned in (iii) is suitable as a method of focusing.
At the time of focusing, a lens is moved. However, not only a lens, but also a mechanical member is moved at the time of focusing. Therefore, it is necessary to secure appropriately a light weight of a lens to be moved, and a space for disposing the mechanical member. As a reflecting telescope optical system in which, the method mentioned in (iii) has been adopted, optical systems described in Japanese Patent Application Laid-open Publication Nos. Hei 5-53058, Sho 63-98618, Sho 60-49313, Sho 51-36133, and Sho 49-66339, and Japanese Patent Publication after Examination Nos. Hei 4-16087 and Hei 3-18162 are available.
Incidentally, in video photography, it is necessary to maintain a focused state by making an auto focus, function all the time. As a method of focusing, a phase-difference auto focus (AF) method and a contrast autofocus method (a so-called mountain-climbing method) are available. In the auto focus in video photography, the contrast AF method has been adopted. In the contrast AF, a change in contrast is measured by moving a focusing lens unit by a minute amount all the time in the vicinity of a focused position. Moreover, a change in the focused state (a shift from the focused state) is detected from the change in the contrast. An operation of moving the focusing lens unit by a minute amount is called as wobbling.
Moreover, in a case in which, the focused state is judged to have been changed; it is possible to bring back the focused state once again by moving the focusing lens unit appropriately. By such wobbling function, it is possible to maintain continuously the focused state all the time even when a distance from an object has changed. However, according to a frame rate which has been set on a camera main-body side, an extremely high-velocity movement is necessary for wobbling, or in other words, for moving the focusing lens unit.